A boosted air system (e.g., turbocharger, supercharger, etc.), as applied to an internal combustion engine, typically introduces noise. For example, a turbocharger's compressor and/or turbine blades may generate whining noises and/or lower frequency rumble. Such disturbances may decrease longevity of a boosted air system and/or other components. In addition, Such disturbances may subjectively annoy people and/or animals in proximity to an operating boosted air system.
In general, noise Occurs as a result of component vibrations and/or aerodynamics (e.g., acoustics). Noise associated with component vibrations may originate from various sources such as bearings. For example, bearings can experience instabilities known as “whirl”, which may depend on a variety of parameters Such as oil pressure, viscosity, and bearing geometry. In particular, journal bearings, which are used often in turbochargers, have one or more oil layers that separate a rotating shaft (or journal) and a fixed housing. Such a bearing geometry has proven susceptible to subsynchronous whirl having large amplitude vibrations between the journal and the housing. Large amplitude vibrations may induce fatigue and reduce considerably component longevity.
Acoustic noise stems from changes in pressure generated, for example, by component vibration and/or aerodynamics. Pressure fluctuations associated with acoustic noise generally travel as longitudinal waves, through air and/or other media. Characteristics of acoustic noise may change abruptly at interfaces between media due to differences in impedance. Of course, a variety of other parameters may determine characteristics of acoustic noise as well.
In vibration noise and/or acoustic noise, frequency normally depends on the number of vibrations or pressure fluctuations over time and noise often contains a large mixture of frequencies at a variety of amplitude levels. As described in more detail below, knowledge of frequency and/or amplitude of noise aids in active control of such noise.